Within flexible alternating current transmission systems (FACTS) a plurality of control apparatus are known. One such FACTS apparatus is a static compensator (STATCOM). A STATCOM comprises a voltage source converter (VSC) having an AC side connected to a high voltage electrical power system and a DC side connected to a temporary electric power storage means such as capacitors. The STATCOM can supply reactive power to or absorb reactive power from the transmission line.
As opposed to the STATCOM, which only compensates for reactive power, another concept is to connect a dc power source to a STATCOM, and thus being able to perform active power compensation. Such a power apparatus may be used e.g. as a spinning reserve and for compensating for fluctuating energy levels in the power system.
Today the dc power source is a high voltage battery. Since the power apparatus is connected to the ac voltage of the high voltage electrical power system, a large number of battery cells have to be connected in series to match the dc voltage of the power apparatus. Moreover, to obtain a desired amount of electric energy (duration of active power) of the energy storage, a number of strings including a plurality of battery cells could be connected in parallel.
In WO/2009/152849 such a power apparatus comprising a voltage source converter and a high voltage dc power source is presented. The power apparatus further comprises solid-state switches which are distributed among the dc power-source units of the string and configured to disconnect the string in case of a fault. By providing each of the dc power source units along the string with a solid-state switch and turning the switches on and off simultaneously, each switch only has to withstand the voltage over its corresponding dc power source unit when the string is disconnected.
Restoring power after a wide-area power outage is a difficult task. A plurality of power stations needs to be brought back on-line. Normally, this is done with the help of power from the rest of the grid. In the absence of grid power, a so-called black start needs to be performed to boot strap the power grid into operation. To provide a black start, some power stations are typically equipped with small diesel generators which can be used to start larger generators, which in turn can be used to start the main power station generators. Generating plants using steam turbines require station service power of up to 10% of their capacity for boiler feedwater pumps, boiler forced-draft combustion air blowers, and for fuel preparation. It is, however, uneconomical to provide such a large standby capacity at each station, so black-start power must be provided over the electrical transmission network from other stations.
An advantage with the power apparatus comprising battery energy storage systems as described above is their ability to provide power to the grid to which it is connected in case of a wide-area power outage making it possible to provide a black start of the grid. In WO/2008/002223 a power compensator is disclosed comprising a voltage source converter, a dc link capacitor and an energy storage device further comprising a high voltage battery, a first and second main switch for disconnecting the battery from the capacitor. One of the switches disconnects the string from the positive dc rail of the converter and the other switch disconnects the string from the negative dc rail of the converter. Each of the switches has to be rated for the entire converter voltage to be capable of protecting the power apparatus. In parallel with the main switches there are arranged a first and a second parallel path containing a resistor means and a secondary switch. The power compensator also comprises a control means for controlling the switches. Further, a method for providing a black start of a dead network is proposed. The method comprises the method steps of:                energizing the dc link capacitor from the energy storage device;        start switching the voltage source converter; and        controlling the power flow of the power compensator to and from the network in dependence on the balance of power producers and power consumers connected to the network.        
However, connecting the energy storage directly to the dc link capacitor would lead to a large inrush current that might result in mechanical or electrical damage to the battery energy storage system and/or converter. It might also lead to a triggering of a short circuit detection system of the power apparatus and consequently a shut down order of the power apparatus. In the method disclosed in WO/2008/002223 and described above this problem is solved by forming a current path containing resistor means in order to decrease the current flow between the dc link capacitor and the energy storage device. However, the resistors are expensive, space requiring, non-controllable and run the risk of becoming overheated.